1. Field of the Invention
The present invention relates generally to a bed for filtering contaminants from liquid and more particularly to a multilayer deep bed filter which includes one or more layers of synthetic, water-wetted, granular filter medium.
2. The Prior Art
As is known to those in the art, sand, anthracite and other materials have been used in mono-media filters for many years with fair results. In more recent years, filtration technology has provided many variations in deep bed filters in an effort to achieve better effluent clarity and longer filtration runs. These more recent filters include plural media layers, such as disclosed in Duff U.S. Pat. No. 3,436,280, Rice U.S. Pat. No. 3,171,801 and Steward U.S. Pat. No. 3,382,983. The first two of these patents disclose the use of anthracite on top of sand and therefore exhibit certain inherent drawbacks, basically because the surface is not porous enough to allow dirt of floc penetration. This is caused by the residual fines on the surface and by the fact that anthracite is comprised of flat or flaky particles which tend to reduce porosity. The third patent is comprised of several layers of relatively heavy filter materials, making it difficult to expand the layers in a backwash cycle for the removal of contaminants and then reform the beds without substantial intermixing.
U.S. Pat. No. 3,343,680 to Rice discloses a deep bed filter using three different medias where the average number of filter particles increases in the direction of filtration flow. This particular arrangement is difficult to achieve, and therefore many in the filtration art have elected to use the simpler dual bed media filters.
In short, filtration technology to date has not provided the art with a multi-layer deep bed filter which includes one or more layers that are comprised of relatively large, substantially uniformly-sized and shaped, water-wettable granules that will leave an exposed porous surface capable of preventing surface loading. Because of this failure or shortcoming in the art, there remain several drawbacks. First, shorter filtration runs have been required because contaminants accumulate at the entrance to the filter bed and "blind-off" the bed. Second, if non-water-wetted materials are used in the top layers, two adverse results may occur: hydrocarbon contaminants may coat the granules, making their rejuvenation difficult; and air bubbles may attach to the granules, altering the overall specific gravity of the granules so that they might flow out of the filter during a backwashing cycle. Third, the overall efficiency of the deep bed filter has not reached its potential due to limitations on the staged filtration concept as a result of the limitations imposed by the practical available filter materials, these limitations including non-uniformly sized granules and limited variations in the density and shape of naturally-occurring water-wettable materials.